This invention relates to turbulent boundary layer flow control devices and more particularly to micro-geometry changes, that is riblets or longitudinal surface striations, of the wall forming the inner region of the boundary layer and ribbon-like strips, that is large-eddy breakup devices or turbulence manipulators, near the outer edge of the boundary layer which decrease boundary layer turbulence and skin-friction drag.
Various types of devices have been used for flow control over aerodynamic surfaces. These devices serve generally two purposes, either to generate separated flow, or to prevent separation. Boundary layer separation may be prevented by generating vortices in the flow thereby energizing the boundary layer or by blowing or sucking on the boundary layer.
Examples of the first type are spoilers and leading edge strips, such as installed on the Cessna T-37 aircraft and on the Lockheed T-33 aircraft. These devices affect the entire flow field creating macro effects around a section of the wing, usually near the root. These devices provide early stall warning and, in some cases, improved spin recovery characteristics.
The function of the strip or spoiler device is to disrupt either a laminar or turbulent flow sufficiently to cause separation of the flow from the surface. The resulting effect is that drag is greatly increased. This penalty is a side effect necessary to gain the aircraft stability or flight characteristic improvements. Devices of this type are unlike the present invention in that the alteration of the flow field is macro, affecting the entire region around the flow surface, while the present invention is micro in effect, creating changes only within the boundary layer.
In addition, the strip and spoiler devices cause additional turbulence and flow separation resulting in additional drag. By contrast, the present invention reduces turbulence and reduces drag.
A separation control device is the vortex generator, typically used to energize the boundary layer. By this method, high speed air is rolled into the stagnant regions of the boundary layer adding velocity to those regions so that flow separation is delayed. These devices are commonly found on the upper surface of wings just ahead of flaps or other controls where the large turning angles of the flow tend to cause separation.
The function of the vortex generator is to energize the boundary layer and thereby increase its resistance to separation. This increased resistance to separation results in an increase in skin friction drag. The vortex generator has the opposite effect compared to the present invention which reduces boundary layer turbulence and skin friction drag.
Other means of boundary layer separation control are blowing and sucking on the boundary layer in regions susceptible to separation. These methods involve either blowing high energy air into the boundary layer, typically blownflaps, such as used on the McDonnell Douglas Phantom, or sucking off the low speed inner region of the boundary layer. Both of these methods result in higher speed fluid or higher energy levels in the boundary layer where separation would normally occur. The blowing method results in increased turbulence and additional skin friction drag. Blowing is similar in function and result to the vortex generator. The suction method reduces turbulence levels but the suction drag increase causes a net drag increase. Unlike the present invention both the blowing and sucking methods are separation control methods, not skin friction reduction methods.
Accordingly, it is an object of the present invention to provide a means of reducing turbulent boundary layer skin friction drag which will be independent of mechanical power sources and will provide a high degree of reliability.
It is a further object of the present invention to provide flow control within the boundary layer which will reduce turbulence.
Another object of the invention is to control the large scale turbulent eddies in the turbulent boundary layer.
Still another object of the present invention is to reduce aerodynamic noise created by boundary layer turbulence such that a saving in insulation weight can be achieved.
Yet another object of the invention is to reduce pressure fluctuations on the surface thereby reducing fatigue and allowing lighter structures.
A further object of the present invention is to control boundary layer flow by riblet geometry on the flow surface and by large-eddy breakup devices in the outer boundary layer.